1. Field of the Invention
The present invention relates to an interface circuit, and more particularly to a sample-and-hold circuit to work between chips or circuits with different working voltages to provide a sample-and-hold function and protect the chips or circuits with a lower working voltage when operating at a higher voltage.
2. Description of the Prior Art
When a signal travels from a chip or circuit with higher working voltage to a chip or circuit with lower working voltage, there must be an interface circuit between the two chips or circuits to attenuate the signal to the range that the chip or circuit with lower working voltage can tolerate. For example, a pickup head usually works at 5V, and the controller of a DVD player works at 3.3V. If the writing voltage (3.3V-5V) of a pickup head inputs to a sample-and-hold circuit, like a Switched-OP, of a controller directly for a long time, the oxide layer manufactured in 3.3V process at the input terminal of the Switched-OP will break down to destroy the controller eternally. The prior art is shown as FIG. 1. A voltage divider, comprising a first resistor 1 and a second resistor 2, is used to attenuate an input signal of a sample-and-hold circuit 3 down to the level lower than 3.3V to prevent this problem.
But in the above situation, the pickup head has to provide extra current “I” to the voltage divider. The extra current produces extra power consumption and thermal noise. Besides, the bigger the resistors are, the longer the setup time of the input of the Switched-OP is. Then the practical sampling time is decreased. Furthermore, the writing voltage unnecessary to be sampled and the reading voltage necessary to be sampled are both attenuated. The attenuated reading voltage combing with the noise at the back end makes the signal/noise ratio lower.